Ground reference scheme for a memory cell

ABSTRACT

Methods, systems, and devices for operating a ferroelectric memory cell or cells are described. A ground reference scheme may be employed in a digit line voltage sensing operation. A positive voltage may be applied to a memory cell; and after a voltage of the digit line of the cell has reached a threshold, a negative voltage may be applied to cause the digit line voltages to center around ground before a read operation. In another example, a first voltage may be applied to a memory cell and then a second voltage that is equal to an inverse of the first voltage may be applied to a reference capacitor that is in electronic communication with a digit line of the memory cell to cause the digit line voltages to center around ground before a read operation.

BACKGROUND

The following relates generally to memory devices and more specificallyto a ground reference scheme for a ferroelectric memory cell.

Memory devices are widely used to store information in variouselectronic devices such as computers, wireless communication devices,cameras, digital displays, and the like. Information is stored byprogramming different states of a memory device. For example, binarydevices have two states, often denoted by a logic “1” or a logic “0.” Inother systems, more than two states may be stored. To access the storedinformation, the electronic device may read, or sense, the stored statein the memory device. To store information, the electronic device maywrite, or program, the state in the memory device.

Various types of memory devices exist, including random access memory(RAM), read only memory (ROM), dynamic RAM (DRAM), synchronous dynamicRAM (SDRAM), ferroelectric RAM (FeRAM), magnetic RAM (MRAM), resistiveRAM (RRAM), flash memory, and others. Memory devices may be volatile ornon-volatile. Non-volatile memory, e.g., flash memory, can store datafor extended periods of time even in the absence of an external powersource. Volatile memory devices, e.g., DRAM, may lose their stored stateover time unless they are periodically refreshed by an external powersource. A binary memory device may be an example of a volatile memorydevice and may store a logic state by charging or discharging acapacitor. A charged capacitor, however, may become discharged over timethrough leakage currents, resulting in the loss of the storedinformation. Certain features of volatile memory may offer performanceadvantages, such as faster read or write speeds, while features ofnon-volatile memory, such as the ability to store data without periodicrefreshing, may be advantageous.

FeRAM may use similar device architectures as volatile memory but mayhave non-volatile properties due to the use of a ferroelectric capacitoras a storage device. FeRAM devices may thus have improved performancecompared to other non-volatile and volatile memory devices. FeRAMsensing schemes may rely on a non-zero reference voltage to compare witha digit line voltage in order to determine a state stored in a memorycell. Using a non-zero reference voltage may, however, fail toaccommodate variations in the digit line voltage and may furtherintroduce errors in the read operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure herein refers to and includes the following figures:

FIG. 1 illustrates an example memory array that supports a groundreference scheme for a memory cell in accordance with variousembodiments of the present disclosure;

FIG. 2 illustrates an example circuit of a memory cell that supports aground reference scheme in accordance with various embodiments of thepresent disclosure;

FIG. 3 illustrates example hysteresis plots for operating aferroelectric memory cell that supports a ground reference scheme inaccordance with various embodiments of the present disclosure;

FIG. 4 illustrates an example circuit that supports a ground referencescheme for a memory cell in accordance with various embodiments of thepresent disclosure;

FIG. 5 illustrates a timing diagram for a ground reference scheme for amemory cell operated in accordance with various embodiments of thepresent disclosure;

FIG. 6 illustrates an example circuit that supports a ground referencescheme for a memory cell in accordance with various embodiments of thepresent disclosure;

FIG. 7 illustrates a timing diagram for a ground reference scheme for amemory cell operated in accordance with various embodiments of thepresent disclosure;

FIG. 8 illustrates an example ferroelectric memory array that supports aground reference scheme in accordance with various embodiments of thepresent disclosure;

FIG. 9 illustrates a device, including a memory array, that supports aground reference scheme in accordance with various embodiments of thepresent disclosure; and

FIGS. 10-11 are flowcharts that illustrate a method or methods for aground reference scheme for a memory cell in accordance with variousembodiments of the present disclosure.

DETAILED DESCRIPTION

A memory device may use a ground reference scheme to increase thereliability of a digit line voltage sensing operation. Ground referenceschemes describe herein may employ techniques that cause possible readvoltages of a digit line—i.e., voltages representing a logic “1” and alogic “0” of a memory cell—to be centered about ground. Digit linevoltages sensed during a read operation may thus be input to a senseamplifier and compared against a ground reference. In the absence of theground reference scheme, the voltage of the digit line during a readoperation may need to be compared to some non-zero value, which may bedifferent from different cells or arrays, or both. Comparison to anon-zero value may thus result in a read operation that is moresensitive to digit line variations and therefore more error prone.

By way of example, a positive voltage may be applied to a plate of aferroelectric capacitor of a memory cell, and the cell may be selectedto discharge the ferroelectric capacitor to a digit line. Once a certaintime has elapsed, or once the digit line voltage has reached a certainthreshold, a negative voltage may be applied to the plate of theferroelectric capacitor. The application of the negative voltage maycause the digit line voltage to drop so that the two possible values ofthe digit line representative of the logic states of the memory cell(i.e., a logic “1” or “0”) are centered about zero volts. The voltage ofthe digit line may then be read and compared with a reference voltageequal to zero volts. For example, a positive digit line voltage mayrepresent a logic “1,” while a negative digit line voltage may representa logic “0.”

The benefits of a ground reference scheme may also be realized without aneed for a negative voltage source. For example, a reference circuit,including a reference capacitor, may be used with a memory cell. Thememory cell may be selected and a voltage applied to the plate of acapacitor of the memory cell at the same time (or nearly the same time)an inverse of the voltage may be applied to the reference capacitor. Thecharge stored in the cell may be transferred to the digit line while thereference capacitor may draw stored charge off the digit line. So, asdescribed below, the reference capacitor may remove a charge from thedigit line and cause the two possible values of the digit line therepresent the logic states of the memory cell (i.e., a logic “1” or “0”)to be centered about zero volts. The benefits of the ground referencescheme discussed above may thus be realized.

Features of the disclosure introduced above are further described belowin the context of a memory array. Specific examples are then describedwith reference to circuits that may be used for a ground referencescheme. These and other features of the disclosure are furtherillustrated by and described with reference to apparatus diagrams,system diagrams, and flowcharts that relate to a ground reference schemeor schemes for a memory cell.

FIG. 1 illustrates an example memory array 100 that supports a groundreference scheme for a memory cell in accordance with variousembodiments of the present disclosure. Memory array 100 may also bereferred to as an electronic memory apparatus. Memory array 100 includesmemory cells 105 that are programmable to store different states. Eachmemory cell 105 may be programmable to store two states, denoted as alogic “0” and a logic “1.” In some cases, memory cell 105 is configuredto store more than two logic states. A memory cell 105 may include acapacitor to store a charge representative of the programmable states;for example, a charged and uncharged capacitor may represent two logicstates, respectively. DRAM architectures may commonly use such a design,and the capacitor employed may include a dielectric material with linearelectric polarization properties. By contrast, a ferroelectric memorycell may include a capacitor that has a ferroelectric as the dielectricmaterial. Different levels of charge of a ferroelectric capacitor mayrepresent different logic states. Ferroelectric materials havenon-linear polarization properties; some details and advantages of aferroelectric memory cell 105 are discussed below.

Operations such as reading and writing may be performed on memory cells105 by activating or selecting the appropriate word line 110 or digitline 115, which may also be referred to as access lines. Activating orselecting a word line 110 or a digit line 115 may include applying avoltage to the respective line. In some cases, a digit line 115 may bereferred to as a bit line. Word lines 110 and digit lines 115 are madeof conductive materials. For example, word lines 110 and digit lines 115may be made of metals, such as copper, aluminum, gold, tungsten, or thelike. According to the example of FIG. 1, each row of memory cells 105are connected to a single word line 110, and each column of memory cells105 are connected to a single digit line 115. By activating (i.e.,applying a voltage to) one of the word lines 110 and one of the digitlines 115, a single memory cell 105 may be accessed at theirintersection. The intersection of a word line 110 and digit line 115 maybe referred to as an address of a memory cell.

In some architectures, the logic-storing device of a cell, e.g., acapacitor, may be electrically isolated from the digit line by aselection component. The word line 110 may be connected to and maycontrol the selection component. For example, the selection componentmay be a transistor and the word line 110 may be connected to the gateof the transistor. Activating the word line 110 results in an electricalconnection between the capacitor of a memory cell 105 and itscorresponding digit line 115. The digit line may then be accessed toeither read or write the memory cell 105.

Accessing memory cells 105 may be controlled through a row decoder 120and a column decoder 130. In some examples, a row decoder 120 receives arow address from the memory controller 140 and activates the appropriateword line 110 based on the received row address. Similarly, a columndecoder 130 receives a column address from the memory controller 140 andactivates the appropriate digit line 115. Thus, by activating a wordline 110 and a digit line 115, memory cell 105 may be accessed. Forexample, the memory array 100 may access memory cell 105 by activatingDL_1 and WL_3.

Upon accessing, memory cell 105 may be read, or sensed, by sensecomponent 125 to determine the stored state of memory cell 105. Forexample, after accessing the memory cell 105, the ferroelectriccapacitor of memory cell 105 may discharge onto the corresponding digitline 115, inducing a voltage on the digit line 115. The voltage of thedigit line 115 may be input to the sense component 125, where thevoltage of the digit line 115 may be compared with a reference voltage.With respect to a memory cell 105 that includes a ferroelectriccapacitor, reading the memory cell may include biasing—e.g., applying avoltage to—a plate of the ferroelectric capacitor.

Sense component 125 may include various transistors or amplifiers todetect and amplify a difference in the signals, which may be referred toas latching. Sense component 125 may include a sense amplifier thatreceives and compares a voltage of a digit line 115 and a referencevoltage. The output of the sense amplifier may be driven to a higher(e.g., a positive) or lower (e.g., negative or ground) supply voltagebased at least in part on the comparison. For instance, if digit line115 has a higher voltage than the reference voltage, then the senseamplifier output may be driven to a positive supply voltage. In somecases, the sense amplifier may additionally drive the output of thedigit line 115 to the supply voltage. Sense component 125 may then latchthe output of the sense amplifier and/or the voltage of digit line 115,which may be used to determine that the stored state in memory cell 105was a logic “1.” Alternatively, if digit line 115 has a lower voltagethan the reference voltage, the sense amplifier output may be driven toa negative or ground voltage. Sense component 125 may then latch theoutput of the sense amplifier, which may be used to determine that thestored state in memory cell 105 was a logic “0.” The detected logicstate of memory cell 105 may then be output through column decoder 130as output 135.

The memory array 100 may use any, or nearly any, voltage as a referencevoltage, and sense component 125 may compare a voltage of a digit line115 to the reference voltage to determine a logic state of memory cell105. But the magnitude of the voltage on the digit line 115 caused byselecting memory cell 105 may vary based on a variety of factors,including the stored state (i.e., logic “1” or logic “0”),characteristics of the ferroelectric capacitor, an applied read voltage,and the like. Because of these variations, a sensed voltage may berelatively close in magnitude to the reference voltage, which maydecrease the sensing margin (i.e., the “margin” between a digit linevoltage that represents a logic “1” or “0” and a reference voltage).This may cause read circuitry to be more sensitive, and thus morecomplex, in order to accurately read the state of memory cell 105; or anarrow sensing margin may increase read errors. Furthermore, there maybe an error in the voltage reference itself. For instance, variations ina supply voltage, temperature, characteristics of a digit line 115 usedas a reference line (e.g., length, trace width, etc.), characteristicsof the memory cell 105 (e.g., parasitic elements), etc., may affect(e.g., increase or decrease) the magnitude of a voltage on a referenceline. If the reference voltage is created by using other memory cells105, characteristics of a ferroelectric capacitor of the memory cells105 may further affect the resulting voltage that is generated on thereference line.

Additional problems associated with using a non-zero reference mayinclude charging of the reference digit line 115 itself—e.g., parasiticcircuit elements may affect the resulting reference voltage—and errorsin the read voltage applied to a plate of a memory cell 105—e.g., ahigher plate voltage may be associated with increased voltages resultingfrom the different logic states. That is, in some cases a higher platevoltage may be applied to a memory cell 105 to increase the amount ofcharge that is extracted from a ferroelectric capacitor and to increasea sensing window—e.g., the difference between the voltages resultingfrom a logic “1” and a logic “0.” Applying a higher plate voltage may,however, also increase the resulting voltages for both logic statesrelative to a reference voltage. Therefore, the resulting voltages maynot be centered around a generated reference voltage, and the sensingmargin may be decreased.

Using zero volts (0V) as a reference—e.g., a sensing scheme where thereference voltage is at ground or virtual ground—may simplify thesensing operation. As described herein, a sensing scheme that uses 0V asa reference is referred to as a ground reference scheme. A groundreference may produce more accurate results, as compared to using anon-zero voltage reference, with a similar sensing window—e.g., thevoltage difference between the voltage resulting from a logic state “0”and a logic state “1”—and/or sensing margin. For example, in a groundreference scheme, a positive digit line voltage may correspond to onelogic state and a negative digit voltage may correspond to a differentlogic state; and whether a digit line voltage is positive or negativemay be easier to ascertain than whether a digit line voltage is above orbelow some non-zero voltage. A ground reference scheme may also reduceerrors associated with generating a non-zero reference voltage and maynot use additional circuitry for reference voltage generation.Furthermore, using a ground reference may reduce testing associated withinitially selecting a preferred reference voltage, which may vary frommemory array to memory array.

In order to employ a ground reference scheme, the possible read voltagesof the digit line 115 resulting from the different logic states may beadjusted so that the voltages associated with a logic “1” and a logic“0” are centered around ground. Circuits and accompanying methods thatmay be implemented to adjust the voltage of the digit line 115 aredescribed in further detail below. As described with reference to FIGS.4 and 5, in some examples, a negative voltage may be applied to theplate of a ferroelectric cell to adjust the voltage of the digit line115. In other examples, including those described with reference toFIGS. 6 and 7, a reference circuit may be used to adjust the voltage ofthe digit line 115; for instance, inverse, or complementary, voltagesmay be applied to the plate of a ferroelectric cell and a plate of thereference capacitor.

A memory cell 105 may be set, or written, by activating the relevantword line 110 and digit line 115. As discussed above, activating a wordline 110 electrically connects the corresponding row of memory cells 105to their respective digit lines 115. By controlling the relevant digitline 115 while the word line 110 is activated, a memory cell 105 may bewritten—i.e., a logic value may be stored in the memory cell 105. Columndecoder 130 may accept data, for example input 135, to be written to thememory cells 105. In the case of a ferroelectric capacitor, a memorycell 105 is written by applying a voltage across the ferroelectriccapacitor. This process is discussed in more detail below.

In some memory architectures, accessing the memory cell 105 may degradeor destroy the stored logic state and re-write or refresh operations maybe performed to return the original logic state to memory cell 105. InDRAM, for example, the capacitor may be partially or completelydischarged during a sense operation, corrupting the stored logic state.So the logic state may be re-written after a sense operation.Additionally, activating a single word line 110 may result in thedischarge of all memory cells in the row; thus, several or all memorycells 105 in the row may need to be re-written.

Some memory architectures, including DRAM, may lose their stored stateover time unless they are periodically refreshed by an external powersource. For example, a charged capacitor may become discharged over timethrough leakage currents, resulting in the loss of the storedinformation. The refresh rate of these so-called volatile memory devicesmay be relatively high—e.g., tens of refresh operations per second forDRAM—which may result in significant power consumption. Withincreasingly larger memory arrays, increased power consumption mayinhibit the deployment or operation of memory arrays (e.g., powersupplies, heat generation, material limits, etc.), especially for mobiledevices that rely on a finite power source, such as a battery.Ferroelectric memory cells may, however, have beneficial properties thatmay result in improved performance relative to other memoryarchitectures. For example, because ferroelectric memory cells tend tobe less susceptible to degradation of stored charge, a memory array 100that employs ferroelectric memory cells 105 may require fewer or norefresh operations, and may thus require less power to operate.

The memory controller 140 may control the operation (e.g., read, write,re-write, refresh, etc.) of memory cells 105 through the variouscomponents, for example, row decoder 120, column decoder 130, and sensecomponent 125. Memory controller 140 may generate row and column addresssignals in order to activate the desired word line 110 and digit line115. Memory controller 140 may also generate and control various voltagepotentials used during the operation of memory array 100. In general,the amplitude, shape, or duration of an applied voltage discussed hereinmay be adjusted or varied and may be different for the variousoperations for operating memory array 100. Furthermore, one, multiple,or all memory cells 105 within memory array 100 may be accessedsimultaneously; for example, multiple or all cells of memory array 100may be accessed simultaneously during a reset operation in which allmemory cells 105, or a group of memory cells 105, are set to a singlelogic state.

In some cases, the memory controller 140 may be used to implementfeatures of the ground reference scheme. For instance, the memorycontroller 140 may provide an input to an amplification device that isused to apply a read voltage to the plate of a ferroelectric capacitorin a memory cell 105. In some examples, the memory controller 140 mayprovide a negative voltage to the amplification device, which in turn,may apply the negative voltage to the plate of the ferroelectriccapacitor. In other examples, the memory controller 140 may provideselection voltages to one or more amplification devices to select amemory cell and a reference capacitor, and subsequently may applycomplementary voltage to the plate of the associated ferroelectric celland the plate of the reference capacitor.

FIG. 2 illustrates an example circuit 200 of a memory cell that supportsa ground reference scheme in accordance with various embodiments of thepresent disclosure. Circuit 200 includes a ferroelectric memory cell105-a, word line 110-a (which may also be referred to as access line110-a), digit line 115-a, and sense component 125-a, which may beexamples of a memory cell 105, word line 110, digit line 115, and sensecomponent 125, respectively, as described with reference to FIG. 1.Memory cell 105-a may include a logic storage component, such ascapacitor 205, which has a first plate and a second plate that arecapacitively coupled, the first plate may be referred to as cell plate230 and the second plate may be referred to as cell bottom 215. In someexamples, the orientation of the capacitor may be flipped withoutchanging the operation of memory cell 105-a, that is, the first platemay correspond to cell bottom 215 and the second plate may correspond tocell plate 230. In the example of FIG. 2, cell plate 230 may be accessedvia plate line 210 and cell bottom 215 may be accessed via digit line115-a. Also in the example of FIG. 2, the terminals of capacitor 205 areseparated by an insulating ferroelectric material. As described above,various states may be stored by charging or discharging capacitor 205,i.e., polarizing the ferroelectric material of capacitor 205. The totalcharge needed to polarize capacitor 205 may be referred to as theremnant polarization (PR) value, and a voltage of capacitor 205 at whichhalf the total charge of capacitor 205 is reached may be referred to asthe coercive voltage (VC).

The stored state of capacitor 205 can be read or sensed by operatingvarious elements represented in circuit 200. Capacitor 205 may be inelectronic communication with digit line 115-a. For example, capacitor205 can be isolated from the digit line 115-a when selection component220 is deactivated and connected to digit line 115-a when selectioncomponent 220 is activated to select the ferroelectric memory cell105-a. In other words, ferroelectric memory cell 105-a may be selectedusing selection component 220 that is in electronic communication withferroelectric capacitor 205, where ferroelectric memory cell 105-aincludes selection component 220 and ferroelectric capacitor 205. Insome cases, selection component 220 is a transistor and its operation iscontrolled by applying a voltage to the transistor gate, where thevoltage magnitude is greater than the threshold magnitude of thetransistor. Word line 110-a may activate selection component 220; forexample, a voltage applied to word line 110-a is applied to thetransistor gate, connecting capacitor 205 with digit line 115-a. In analternative embodiment the positions of selection component 220 andcapacitor 205 may be switched, such that selection component 220 isbetween plate line 210 and cell plate 230 and such that capacitor 205 isbetween digit line 115-a and the other terminal of selection component220. In this embodiment, selection component 220 may remain inelectronic communication digit line 115-a through capacitor 205. Thisconfiguration may be associated with alternative timing and biasing forread and write operations.

Due to the ferroelectric material between the plates of capacitor 205,and as discussed in more detail below, capacitor 205 may not dischargeupon connection to digit line 115-a. In one scheme, to sense the statethat is stored by ferroelectric capacitor 205 during a read, plate line210 and word line 110-a may be biased by an external voltage. In somecases, digit line 115-a is isolated from a virtually ground prior toapplying the external voltage to plate line 210 and word line 110-a.Selecting ferroelectric memory cell 105-a may result in a voltagedifference (e.g., plate line 210 voltage minus digit line 115-a voltage)across capacitor 205. The applied voltage difference may yield a changein the stored charge on capacitor 205, which may depend on the initialstate of capacitor 205—e.g., whether the initial state stored a logic“1” or a logic “0”—and may induce a voltage on digit line 115-a based onthe resulting charge stored on capacitor 205. The induced voltage ondigit line 115-a may then be compared to a reference (e.g., a voltage ofreference line 225) by sense component 125-a in order to determine thestored logic state in memory cell 105-a.

The specific sensing scheme or process may take many forms. In oneexample, digit line 115-a may have an intrinsic capacitance and developa non-zero voltage as capacitor 205 charges or discharges in response tothe voltage applied to plate line 210. The intrinsic capacitance maydepend on physical characteristics, including the dimensions, of digitline 115-a. Digit line 115-a may connect many memory cells 105 so digitline 115-a may have a length that results in a non-negligiblecapacitance (e.g., on the order of pico farads (pF)). The subsequentvoltage of digit line 115-a may depend on the initial logic state ofcapacitor 205, and sense component 125-a may compare this voltage to avoltage on reference line 225 provided by a reference component. Circuit200 may be operated so that the possible digit line 115 voltages duringa read operation are centered about 0V. That is, a negative voltage maybe applied to plate line 210 or a reference circuit (not shown) may beemployed to effectively lower voltage of digit line 115 so that it canbe compared with ground during a read operation.

In some examples, the reference line 225 is an unused digit line, whichmay be grounded. For example, a voltage may be applied to plate line 210and a voltage at capacitor bottom 215 may change in relation to thestored charge. The voltage at capacitor bottom 215 may be compared witha reference voltage at sense component 125-a, and a comparison to thereference voltage may indicate a change in the charge of capacitor 205resulting from the applied voltage and thus indicate a logic statestored in memory cell 105-a. The reference voltage may be 0V (i.e.,ground or virtual ground). The relationship between charge and voltagein capacitor 205 is described in further detail with reference to FIG.3.

To write memory cell 105-a, a voltage may be applied across capacitor205. Various methods may be used. In one example, selection component220 may be activated through word line 110-a in order to electricallyconnect capacitor 205 to digit line 115-a. A voltage may be appliedacross capacitor 205 by controlling the voltage of cell plate 230 usingplate line 210 and controlling the voltage of cell bottom 215 usingdigit line 115-a. To write a logic “0,” cell plate 230 may be takenhigh, that is, a positive voltage may be applied to plate line 210, andcell bottom 215 may be taken low—e.g., virtually grounded using digitline 115-a. The opposite process is performed to write a logic “1”,i.e., cell plate 230 may be taken low and cell bottom 215 may be takenhigh. Read and write operations of capacitor 205 may account for thenon-linear properties associated with a ferroelectric device.

FIG. 3 illustrates examples of such non-linear properties withhysteresis curves 300-a and 300-b for a memory cell that supports aground reference scheme in accordance with various embodiments of thepresent disclosure. Hysteresis curves 300-a and 300-b illustrate anexample ferroelectric memory cell writing and reading process,respectively. Hysteresis curves 300 depict the charge, Q, stored on aferroelectric capacitor (e.g., capacitor 205 of FIG. 2) as a function ofa voltage, V.

A ferroelectric material is characterized by a spontaneous electricpolarization, i.e., it maintains a non-zero electric polarization in theabsence of an electric field. Example ferroelectric materials includebarium titanate (BaTiO₃), lead titanate (PbTiO₃), lead zirconiumtitanate (PZT), and strontium bismuth tantalate (SBT). The ferroelectriccapacitors described herein may include these or other ferroelectricmaterials. Electric polarization within a ferroelectric capacitorresults in a net charge at the ferroelectric material's surface andattracts opposite charge through the capacitor terminals. Thus, chargeis stored at the interface of the ferroelectric material and thecapacitor terminals. Because the electric polarization may be maintainedin the absence of an externally applied electric field for relativelylong times, even indefinitely, charge leakage may be significantlydecreased as compared with, for example, capacitors employed in DRAMarrays. This may reduce the need to perform refresh operations asdescribed above for some DRAM architectures.

Hysteresis curves 300 may be understood from the perspective of a singleterminal of a capacitor. By way of example, if the ferroelectricmaterial has a negative polarization, positive charge will accumulate atthe terminal. Likewise, if the ferroelectric material has a positivepolarization, negative charge will accumulate at the terminal.Additionally, it should be understood that the voltages in hysteresiscurves 300 represent a voltage difference across the capacitor and aredirectional. For example, a positive voltage may be applied by applyinga positive voltage to the terminal in question and maintaining thesecond terminal at ground. A negative voltage may be applied bymaintaining the terminal in question at ground and applying a positivevoltage to the second terminal—i.e., positive voltages may be applied tonegatively polarize the terminal in question. Similarly, two positivevoltages, two negative voltages, or any combination of positive andnegative voltages may be applied to the appropriate capacitor terminalsto generate the voltage difference shown in hysteresis curves 300.

As depicted in hysteresis curve 300-a, the ferroelectric material maymaintain a positive or negative polarization with a zero voltagedifference, resulting in two possible charged states: charge state 305and charge state 310. According to the example of FIG. 3, charge state305 represents a logic “0” and charge state 310 represents a logic “1.”In some examples, the logic values of the respective charge states maybe reversed to accommodate other schemes for operating a memory cell.

A logic “0” or “1” may be written to the memory cell by controlling theelectric polarization of the ferroelectric material, and thus the chargeon the capacitor terminals, by applying voltage. For example, applying anet positive voltage 315 across the capacitor results in chargeaccumulation until charge state 305-a is reached. Upon removing voltage315, charge state 305-a follows path 320 until it reaches charge state305 at zero voltage potential. Similarly, charge state 310 is written byapplying a net negative voltage 325, which results in charge state310-a. After removing negative voltage 325, charge state 310-a followspath 330 until it reaches charge state 310 at zero voltage.

To read, or sense, the stored state of the ferroelectric capacitor, avoltage may be applied across the capacitor. In response, the storedcharge changes, and the degree of the change depends on the initialcharge state—i.e., the degree to which the stored charge of thecapacitor changes varies depending on whether charge state 305-b or310-b was initially stored. For example, hysteresis curve 300-billustrates two possible stored charge states 305-b and 310-b. Netvoltage 335 may be applied to the cell plate (e.g., cell plate 230 withreference to FIG. 2) of the capacitor. Although depicted as a positivevoltage, voltage 335 may be negative. In response to voltage 335, chargestate 305-b may follow path 340. Likewise, if charge state 310-b wasinitially stored, then it follows path 345. The final position of chargestate 305-c and charge state 310-c depend on a number of factors,including the specific sensing operation and circuitry.

In some cases, the final charge may depend on the intrinsic capacitanceof the digit line of a memory cell. For example, if the capacitor iselectrically connected to the digit line and voltage 335 is applied, thevoltage of the digit line may rise due to its intrinsic capacitance, andthe voltage measured at a sense component may depend on the resultingvoltage of the digit line. The position of final charge states 305-c and310-c on hysteresis curve 300-b may thus depend on the capacitance ofthe digit line and may be determined through a load-line analysis—i.e.,charge states 305-c and 310-c may be defined with respect to the digitline capacitance. As a result, the voltage of the capacitor, voltage 350or voltage 355, may be different and may depend on the initial state ofthe capacitor.

By comparing the difference of the voltage applied to the cell plate(e.g., voltage 335) and the voltage across the capacitor (e.g., voltage350 or voltage 355) to a reference voltage, the initial state of thecapacitor may be determined. As can be understood by reference to FIG.2, the voltage of the digit line may be represented as the difference ofthe voltage applied to plate line 210 and the resulting voltage acrossthe capacitor 205. As discussed above, the voltage of the digit line isbased at least in part on the change in charge stored at the capacitor,and the change in charge is associated with the magnitude of the voltagethat is applied across the capacitor. In some examples, the referencevoltage may be an average of the digit line voltages that result fromvoltage 350 and 355 and, upon comparison, the sensed digit line voltagemay be determined to be higher or lower than the reference voltage. Avalue of the ferroelectric cell (i.e., a logic “0” or “1”) may then bedetermined based on the comparison.

However, as discussed above, the digit line and reference voltages mayvary based at least in part on cell characteristics (e.g., age),environmental factors (e.g., temperature), applied plate voltage, etc.In certain scenarios, using the average of the digit line voltages asthe reference voltage may decrease the sensing margin. For instance,variations in the digit line voltages resulting from two logical statesmay increase the average of the digit line voltages and the referencevoltage may be biased towards one of the digit line voltages. Using aground reference in place of a non-zero voltage reference (e.g., theaverage of the digit line voltages) may reduce errors associated withthe reference voltage, simplify the generation of the reference voltage,and reduce complexity associated with sensing operations.

As discussed above, reading a memory cell that does not use aferroelectric capacitor may degrade or destroy the stored logic state. Aferroelectric memory cell, however, may maintain the initial logic stateafter a read operation. For example, if charge state 305-b is stored andthe read operation performed, the charge state may follow path 340 tocharge state 305-c, and after removing voltage 335, the charge state mayreturn to initial charge state 305-b, for example, by following path 340in the opposite direction.

FIG. 4 illustrates an example circuit 400 that supports a groundreference scheme for a memory cell in accordance with variousembodiments of the present disclosure. Circuit 400 includes memory cell105-b, word line 110-b (which may also be referred to as access line110-b), digit line 115-b, and sense component 125-b, which may beexamples of a memory cell 105, word line 110, digit line 115, and sensecomponent 125, respectively, described with reference to FIGS. 1 and 2.Circuit 400 also includes plate line 210-a and reference line 225-a,which may be examples of plate line 210 and reference line 225,respectively, described with reference to FIG. 2. Circuit 400 alsoincludes voltage source 405, voltage source 410, and switching component420.

Digit line 115-b and reference line 225-a may have respective intrinsiccapacitances 415-a and 415-b, respectively. Intrinsic capacitances 415-aand 415-b may not be electrical devices—i.e., may not be two-terminalcapacitors. Instead, intrinsic capacitances 415-a and 415-b may dependon physical characteristics, including the dimensions, of digit line115-b and reference line 225-a. In some cases, reference line 225-a isan unused or inactive digit line. In some examples, although notdepicted, digit line 115-b is connected to a virtual ground via aswitching component. A virtual ground may act as a common reference forcircuit 400 and may also be referred to as ground or 0V, although, thevirtual ground may float to a voltage that is different than (e.g.,greater or less than) zero volts when compared with an earth ground.

The voltage of reference line 225-a may be used by sense component 125-bas a reference for comparison against the voltage of digit line 115-b.In some examples, reference line 225-a is connected to a virtual groundto provide a ground reference for comparing with the voltage of digitline 115-b. The reference line 225-a may be separated from virtualground through a switching component 420, which may be implemented as atransistor (e.g., a p-type field effect transistor (FET)). In othercases, reference line 225-a may be directly connected to the virtualground.

As depicted, memory cell 105-b is in electronic communication with digitline 115-b. Memory cell 105-b may be selected using a selectioncomponent that is in electronic communication with a ferroelectriccapacitor via word line 110-b, as described with reference to FIG. 2.Activating the selection component may connect the ferroelectriccapacitor to digit line 115-b.

The plate line 210-a may be in electronic communication with theferroelectric capacitor (e.g., a plate of the ferroelectric capacitor).To read memory cell 105-b, a voltage may be applied to plate line 210-aof the ferroelectric capacitor of memory cell 105-b. Applying a positivevoltage to plate line 210-a in combination with applying a voltage toword line 110-b may result in the ferroelectric capacitor charging digitline 115-b. After applying the positive voltage, a negative voltage maybe applied to the plate line 210-a to adjust the voltage of digit line115-b. In some cases, the negative voltage is applied after determiningthat the voltage of digit line 115-b has reached a threshold in responseto the applied positive voltage. The negative voltage may be selected toadjust a voltage resulting from a logic state “0” stored in theferroelectric capacitor and the voltage resulting from a stored logicstate “1” to be centered around virtual ground. By way of example,ferroelectric memory cells from a ferroelectric memory array may betested—e.g., by applying varying plate voltages, temperatures, etc.—todetermine an average logic “1” voltage and an average logic “0” voltageand the negative voltage may be selected accordingly. In other cases,the negative voltage may be selected based on a mathematical model thathas been developed for a ferroelectric memory array or based onestablished test results. In some cases, a voltage may be applied toplate line 210-a and word line 110-a via an external voltage source,amplifier, a line driver, or the like.

Sense component 125-b may be used to determine the stored state ofmemory cell 105-b. In some cases, sense component 125-b may be or mayinclude a sense amplifier. Sense component 125-b may be operated byvoltage source 405 and voltage source 410. In some examples, voltagesource 405 is a positive supply voltage, while voltage source 410 is anegative supply voltage or a virtual ground. Sense component 125-b maybe used to determine a logic value of the ferroelectric memory cell105-b based at least in part on the voltage of digit line 115-b and thevoltage of the reference line 225-a. Sense component 125-b may beactivated or deactivated by a controller. In some cases, sense component125-b is activated or “fired” to trigger a comparison between thevoltage of digit line 115-b and the voltage of reference line 225-a.Sense component 125-b may latch the output of a sense amplifier to thevoltage provided by either voltage source 405 or voltage source 410. Forinstance, if the voltage of the digit line 115-b is greater than thevoltage of the reference line 225-a, then sense component 125-b maylatch the output of the sense amplifier at a positive voltage suppliedfrom voltage source 405.

FIG. 5 illustrates a timing diagram 500 for a ground reference schemefor a memory cell in accordance with various embodiments of the presentdisclosure. Timing diagram 500 depicts voltage on axis 505 and time onaxis 510. The voltage of various components as a function of time maythus be represented on timing diagram 500. For example, timing diagram500 includes word line voltage 515, plate voltage 520, and digit linevoltages 530-a and 530-b. Timing diagram 500 also includes a readvoltage 535, a voltage threshold 540, a reference voltage 545, and atiming threshold 550. Timing diagram 500 depicts an example operation ofcircuit 400 described with reference to FIG. 4. FIG. 5 is describedbelow with reference to components of preceding figures. Voltages thatapproach zero may be offset from axis 510 for ease of representation; insome cases, these voltages may be equal to or substantially equal tozero.

As discussed in FIG. 4, a voltage may be applied to the plate line210-a. In some examples, a read voltage—i.e., the voltage used to readthe state of a ferroelectric capacitor, may be applied to plate line210-a, biasing the ferroelectric capacitor. The plate voltage 520, whichmay be measured at the plate of the ferroelectric capacitor, may risewith the applied read voltage. After applying the read voltage, memorycell 105-b may be accessed by applying another voltage to word line110-b. The word line voltage 515, which may be measured at the gate of aselection component of memory cell 105-a, may rise with the voltageapplied to word line 110-b. As the word line voltage 515 rises, theselection component may provide a conductive path between the biasedferroelectric capacitor of memory cell 105-b and digit line 115-b.Accordingly, the digit line voltage 530 may rise as the ferroelectriccapacitor discharges onto digit line 115-b. The digit line voltage 530may be virtually grounded before a read operation is performed. In oneexample, a switching component, such as a transistor, may be used toconnect the digit line 115-c to ground.

Upon selecting the word line 110-b, the ferroelectric capacitor ofmemory cell 105-b shares charge with the intrinsic capacitance 415-auntil the voltage of the cell bottom (e.g., cell bottom 215 as describedwith reference to FIG. 2) of the ferroelectric capacitor of memory cell105-b and the voltage across intrinsic capacitance 415-a are equal. Thedigit line voltage 530 may rise to one of two voltages based on thestored state. Although, as discussed above, these two voltages may varywith characteristics of the memory cell 105-b, temperature, etc. If alogic “1” is stored by the ferroelectric capacitor, then digit linevoltage 530-a may result, while digit line voltage 530-b may result if alogic “0” is stored. Digit line voltage 530-a may be associated with asmaller voltage drop over the ferroelectric cell and therefore a higherdigit line voltage when compared with digit line voltage 530-b, asdescribed with reference to FIG. 3.

According to the example depicted in FIG. 5, after the digit linevoltage 530 reaches a voltage threshold or a timing threshold 550, orboth, a negative voltage is applied to plate line 210-a, driving platevoltage 520 negative. In some cases, the negative voltage may be appliedafter determining that the positive voltage has been applied for apredetermined duration—e.g., for a duration that exceeds timingthreshold 550. The predetermined duration may be determined based atleast in part on a characteristic of the ferroelectric capacitor, acharacteristic of the digit line, a timing associated with reading orwriting to the ferroelectric memory cell, or any combination thereof. Inother cases, the negative voltage may be applied based on determiningthat the digit line has reached a threshold value. Or in some cases, thenegative voltage is applied based on determining that the digit linevoltage 530 has settled—e.g., that the rate of change of the voltage ofthe digit line has reached a threshold. For instance, it may bedetermined that the rate of change is lower than a threshold value(e.g., less than 10 mV/ns). In other examples, the negative voltage maybe applied based on determining that the digit line voltage 530 iswithin a percent range (e.g., within 5%) of an expected settling voltageof either the digit line voltage 530-a or 530-b. In some cases, theexpected settling voltage may be determined using experimental data, aprediction model, or the like.

As plate voltage 520 transitions to a negative voltage, the digit linevoltage 530 may return charge to the ferroelectric capacitor and thedigit line voltage 530 may also decrease. The decrease in the digit linevoltage 530 may be dependent upon the magnitude of the applied negative.In some cases, the magnitude of the negative voltage is selected basedat least in part on the resulting decrease in digit line voltages 530-aand 530-b. In some examples, the negative voltage is selected to centerthe digit line voltages 530-a and 530-b—i.e., the possible digit linevoltages resulting from selecting the memory cell—around a referencevoltage 545, which may be at a virtual ground (e.g., 0V as shown intiming diagram 500). In some cases, reference voltage 545 may begenerated by connecting reference line 225-a to virtual ground viaswitching component 420. After the digit line voltages 530 resultingfrom the applied negative voltage have settled, sense component 125-b isfired at time 555.

The sense component 125-b may compare the digit line voltage 530 withthe reference voltage 545 and the output of the sense component 125-bmay be latched, accordingly. For instance, if a logic value “1” isstored by the ferroelectric capacitor, then the sense component 125-bmay compare digit line voltage 530-a with reference voltage 545 and maydetermine the digit line voltage 530-a is higher than the referencevoltage 545. Therefore, the output of the sense component 125-b may bedriven to a positive supply voltage and latched. In this example, whenthe sense component 125-b outputs the positive supply voltage, digitline 115-b is also driven to the supply voltage.

FIG. 6 illustrates an example circuit 600 that supports a groundreference scheme for a memory cell in accordance with variousembodiments of the present disclosure. Circuit 600 includes memory cell105-c, word line 110-c (which may also referred to as access line110-c), digit line 115-c, and sense component 125-c, which may beexamples of memory cell 105, word line 110, digit line 115, and sensecomponent 125, respectively, described with reference to FIGS. 1, 2, 4,and 5. Circuit 600 may also include plate line 210-b, and reference line225-b, which may be examples of plate line 210 and reference line 225,respectively, described with reference to FIGS. 2 and 4. Additionally,circuit 600 includes voltage source 405-a, voltage source 410-a,intrinsic capacitances 415-c and 415-d, and switching components 420-aand 420-b, which may be examples of voltage source 405, voltage source410, and intrinsic capacitances 415, and switching component 420described with reference to FIG. 4. Circuit 600 may also includereference circuit 605, which may be in electronic communication withdigit line 115-c, and which may include reference capacitor 615,selection component 610, selection line 620, and reference plate line625.

Selection component 610 may be used to connect digit line 115-c andreference capacitor 615. Reference capacitor 615 may be implemented as adielectric, ceramic, electrolytic, or a ferroelectric capacitor. In somecases, selection component 610 may be a transistor, such as a p-typeFET. Selection line 620 may be in electronic communication with and usedto activate selection component 610. For example, reference capacitor615 may be accessed by applying a voltage to selection line 620, which,in the case of a ferroelectric capacitor, may be implemented as a wordline. The size of reference capacitor 615 may be selected so that thevoltages of digit line 115-c that result from a logic “0” and a logic“1” are centered around a ground reference. In one example, the size ofthe capacitor is chosen to be approximately 80 femtofarads (fF). In somecases, the size of the reference capacitor 615 may be selected to be theaverage of the capacitance associated with a ferroelectric capacitorstoring a logic “0” and the capacitance associated with a ferroelectriccapacitor storing a logic “1”. In another example, if a ferroelectriccapacitor is used, the size of the ferroelectric capacitor may be chosento be greater than the size of the ferroelectric capacitor of memorycell 105-c. In some examples, the reference capacitor may be implementedby using a first ferroelectric capacitor storing a logic “0” and asecond ferroelectric capacitor storing a logic “0.”

Reference plate line 625 may be in electronic communication withreference capacitor 615 and digit line 115-c. Initially, a positivevoltage may be applied to reference plate line 625 and a zero voltagemay be applied to plate line 210-b to bias a ferroelectric capacitor ofmemory cell 105-c and to bias reference capacitor 615. In some cases,the other side of reference capacitor 615 may be maintained at a groundreference via switching component 420-b to enable charging of referencecapacitor 615. Subsequently, selection voltages may be applied to wordline 110-c and selection line 620 to access memory cell 105-c andreference circuit 605. Applying the selection voltages to word line110-c and selection line 620 may connect the ferroelectric capacitor ofmemory cell 105-c and reference capacitor 615 to digit line 115-c,respectively. In some cases, digit line 115-c may maintain a connectionto virtual ground for a duration after the selection voltages have beenapplied, and may subsequently be isolated from ground. At a later pointin time, a zero voltage may be applied to reference plate line 625 and apositive voltage may be applied to plate line 210-b. In some cases, themagnitude of the positive voltage applied to the plate line 210-b may bedifferent than the magnitude of the positive voltage previously appliedto the reference capacitor 615.

Applying the inverse, or complementary, voltages (i.e., the zero voltageto reference plate line 625 and the positive voltage to plate line210-b) may cause the ferroelectric capacitor to discharge onto digitline 115-c while reference capacitor 615 draws charge from digit line115-c. As charge builds up on digit line 115-c, the voltage of digitline 115-c may increase; and as charge is drawn from digit line 115-c,the voltage of digit line 115-c may decrease. These complementaryfunctions may be used to center the resulting logic “1” and logic “0”voltages—i.e., the possible voltages of digit line 115-c that resultfrom selecting memory cell 105-c—around zero volts. The voltage of digitline 115-c may be compared with the voltage of reference line 225-b todetermine the logic value that is stored by memory cell 105-c. In somecases, the voltage of reference line 225-b may be at virtual ground, asdiscussed above. In some cases, a voltage may be applied to plate line210-b, word line 110-b, selection line 620, and reference plate line625, via an external voltage source, an amplifier, a line driver, or thelike.

FIG. 7 illustrates a timing diagram 700 that supports a ground referencescheme for a memory cell in accordance with various embodiments of thepresent disclosure. Timing diagram 700 depicts voltage on axis 505-a andtime on axis 510-a. The voltage of various components as a function oftime are represented on timing diagram 700. For example, timing diagram700 includes word line voltage 515-a, plate voltage 520-a, and digitline voltages 530-c and 530-d, which may be examples of word linevoltage 515, plate voltage 520, and digit line voltage 530 describedwith reference to FIG. 5. Timing diagram 700 may also include a readvoltage 535-a and a reference voltage 545-a, which may be examples ofread voltage 535 and reference voltage 545 described with reference toFIG. 5. Timing diagram 700 may result from operating circuit 600described with reference to FIG. 6. FIG. 7 is described below withreference to components of preceding figures. Voltages that approachzero may be offset from axis 510-a for ease of representation, in somecases, these voltages may be equal to or substantially equal to zero.

As discussed with reference to FIG. 6, a reference plate voltage 710 maybe applied to the reference plate line 625 to charge reference capacitor615, while the plate voltage 520-a applied to plate line 210-b and theother side of the reference capacitor may be maintained at virtualground. In some cases, the magnitude of the applied voltage may be avoltage associated with a read operation. Subsequently, selectionvoltage 705 may be applied to selection line 620 to create a conductivepath between reference capacitor 615 and digit line 115-c via selectioncomponent 610. Substantially simultaneously with applying selectionvoltage 705, word line voltage 515-a may be applied to word line 110-cto create a conductive path between a ferroelectric capacitor of memorycell 105-c and digit line 115-c. In some cases, the same voltage is usedto access the ferroelectric capacitor and reference capacitor of bothmemory cell 105-c and reference circuit 605, while in other casesdifferent voltages are used. In some cases, the digit line voltage 530may be maintained at a ground reference via switching component 415-cbefore and after selection voltage 705 and word line voltage 515-a hasbeen applied.

The reference plate voltage 710 and plate voltage 520-a may then beinversely applied—e.g., the voltages may be moved in complementarydirections—to the reference plate line 625 and the plate line 210-b.Accordingly, the reference plate voltage 710 may decrease and the platevoltage 520-a may increase. That is, the voltage applied to referenceplate line 625 may be removed to be zero volts, and, substantiallysimultaneously, a voltage (e.g., a read voltage) may be applied to plateline 210-b. Applying the voltages substantially simultaneously meansapplying the voltages at or about the same time. Applying voltagessubstantially simultaneously may also mean that within a certain timeperiod of applying one voltage, a second voltage is applied—e.g., withinhalf a nanosecond (ns) of applying a first voltage, the second voltageis applied. For instance, the decrease of reference plate voltage 710may overlap with the increase of plate voltage 520-a. In some cases, thetime period between the voltages applied at word line 110-c andreference plate line 625 increases due to characteristics of a memoryarray (e.g., propagation delay). In these cases, the increase anddecrease of reference plate voltage 710 and plate voltage 520-a may benon-overlapping and the time period between applying the voltages may beas large as 3 ns. As depicted, increasing the plate voltage 520-a maycause the ferroelectric capacitor of memory cell 105-c to discharge ontodigit line 115-c, while decreasing the reference plate voltage 710 maypull charge from digit line 115-c.

Providing charge to digit line 115-c may raise the digit line voltage530 and removing charge may lower the digit line voltage 530. The amountof charge removed from digit line 115-c may be associated with the rateof change in reference plate voltage 710, the size of referencecapacitor 615, the charge currently stored by capacitor 615, themagnitude of the reference plate voltage 710, or any combinationthereof. In some cases, the reference plate voltage 710 and referencecapacitor 615 size are selected to center the resulting digit linevoltages 530-c and 530-d around a virtual ground. The sense component125-c may be fired at time 555-a to compare the digit line voltage 530with the reference voltage 545-a. If the sensed digit line voltage 530is high (e.g., digit line voltage 530-c) and compared with reference545-a, then the output of sense component 125-c and the digit linevoltage 530-c may rise to the voltage supplied by voltage source 405-a.Otherwise, if the sensed digit line voltage 530 is low (e.g., digit linevoltage 530-d) and compared with the reference, then the output of thesense component 125-c and the digit line voltage may rise to the voltagesupplied by voltage source 410-a. The output of the sense component125-c may be latched and used to determine the stored state associatedwith memory cell 105-c.

FIG. 8 shows a block diagram 800 of a memory array 100-a that supports aground reference scheme for a memory cell in accordance with variousembodiments of the present disclosure. Memory array 100-a may containmemory controller 140-a and memory cell 105-d, which may be examples ofmemory controller 140 and memory cell 105 described with reference toFIGS. 1 and 2. Memory controller 140-a may include biasing component810, timing component 815, and digit line (DL) voltage adjust component830, and may operate memory array 100-a as described in FIGS. 1-7.

Memory controller 140-a may be in electronic communication with wordline 110-d (which may also referred to as access line 110-d), digit line115-d, sense component 125-d, plate line 210-c, and reference circuit605-a, and memory cell 105-d which may be examples of word line 110,digit line 115, sense component 125, plate line 210, reference circuit605, and memory cell as described with reference to FIG. 1, 2, 4, or 6.Memory array 100-a may also include reference component 820, latch 825,and control line 835. The components of memory array 100-a may be inelectronic communication with each other and may perform the functionsdescribed with reference to FIGS. 1-7. In some cases, referencecomponent 820, sense component 125-a, and latch 825 may be components ofmemory controller 140-a.

Memory controller 140-a may be configured to activate word line 110-d,plate line 210-a, or digit line 115-d by applying voltages to thosevarious nodes. In some cases, memory controller 140-a may perform itsoperations using biasing component 810. For example, biasing component810 may be configured to apply a voltage to operate memory cell 105-d toread or write memory cell 105-d as described above. In some cases,memory controller 140-a may include a row decoder, column decoder, orboth, as described with reference to FIG. 1. This may enable memorycontroller 140-a to access one or more memory cells 105. Biasingcomponent 810 may also provide voltages to reference component 820 inorder to generate a reference signal for sense component 125-a.Additionally, biasing component 810 may provide voltages for theoperation of sense component 125-a.

In some cases, memory controller 140-a may perform its operations usingtiming component 815. For example, timing component 815 may control thetiming of the various word line selections or plate biasing, includingtiming for switching and voltage application to perform the memoryfunctions, such as reading and writing, discussed herein. In some cases,timing component 815 may control the operations of biasing component810. In some cases, timing component 815 may be used to select memorycell 105-d for a read operation and to trigger reference circuit 605-a.

Reference component 820 may include various components to generate areference signal for sense component 125-a. Reference component 820 mayinclude circuitry specifically configured to produce a reference signal.In some cases, reference component 820 may be other ferroelectric memorycells 105. In some examples, reference component 820 may be configuredto output a voltage with a value between the two sense voltages, asdescribed with reference to FIG. 3. Or reference component 820 may bedesigned to output a virtual ground. Sense component 125-a may compare asignal from memory cell 105-d (through digit line 115-d) with areference signal from reference component 820. Upon determining thelogic state, the sense component may then store the output in latch 825,where it may be used in accordance with the operations of an electronicdevice using the memory device of which memory array 100-a is a part.

In some cases, memory controller 140-a may adjust the voltage of digitline 115-d via control line 835. For example, DL voltage adjustcomponent 830 may be used to adjust the voltage of digit line 115-d sothat a ground reference may be utilized. In some cases, DL voltageadjust component 830 may be used to center the digit line voltagesresulting from an initial logic state “1” or “0.” For instance, DLvoltage adjust component 830, in conjunction with biasing component 810and timing component 815, may be used to apply a positive voltage toplate line 210-c, determine that a voltage of digit line 115-d hasreached a threshold, and apply a negative voltage to plate line 210-cafter the voltage of digit line 115-d has reached a threshold. Incertain examples, memory controller 140-a may bias plate line 210-cusing control line 835.

For example, biasing component 810 may connect the ferroelectriccapacitor of memory cell 105-d to a first voltage source (e.g., apositive voltage source) or to a second voltage source (e.g., a negativevoltage source), or both. The timing component 815 and/or DL voltageadjust component 830 may be used to determine that a voltage of digitline 115-d has reached a threshold in response to an applied positivevoltage. In some cases, determining the voltage of the digit line 115-dhas reached a threshold may be based at least in part on determiningthat the positive voltage has been applied for a predetermined duration,determining that the voltage of the digit line has reached a thresholdvoltage, determining that a rate of change of the voltage of the digitline has reached the threshold, or any combination thereof. Memorycontroller 140-a may be used to trigger sense component 125-a to comparethe voltage of the digit line 115-d to a ground reference after thenegative voltage is applied. In some cases, memory controller 140-a mayuse an output of sense component 125-a is used to determine a logicvalue of the ferroelectric memory cell based at least in part on thecomparison of the voltage of the digit line to the ground reference.

In another example, DL voltage adjust component 830 may be used incombination with biasing component 810, timing component 815, andreference circuit 605-a to apply a first voltage to plate line 210-c anda second voltage to reference circuit 605-a, wherein the second voltageis an inverse of the first voltage. In certain examples, memorycontroller 140-a utilizes biasing component 810 and control line 835trigger reference circuit 605-a and to bias plate line 210-c.

For example, biasing component 810 may be used to connect a firstvoltage source to a ferroelectric capacitor of the ferroelectric memorycell 105-d, wherein the ferroelectric capacitor is in electroniccommunication with digit line 115-d via a first selection component.Biasing component 810 may also be used to connect a second voltagesource to a reference capacitor of reference circuit 605-a, wherein thereference capacitor is in electronic communication with the digit linevia a second selection component, wherein the second voltage is aninverse of the first voltage and is applied based at least in part onapplying the first voltage. In some cases, timing component 815 maytrigger the biasing component 810 to apply the first voltage and secondvoltages substantially simultaneously. Biasing component 810 may also beused to activate the first selection component to perform a readoperation of the ferroelectric memory cell and to activate the secondselection component to transfer a charge of the reference capacitor todigit line 115-d during the read operation. Biasing component 810 mayalso be used to virtually ground the digit line before connecting thefirst voltage source to the ferroelectric capacitor or the secondvoltage source to the reference capacitor. As above, sense component125-a may compare a signal from memory cell 105-d (through digit line115-d) with a reference signal from reference component 820. In somecases, memory controller 140-a may use an output of sense component125-a is used to determine a logic value of the ferroelectric memorycell based at least in part on the comparison of the voltage of thedigit line to the ground reference.

FIG. 9 illustrates a system 900 that supports a ground reference schemefor a memory cell in accordance with various embodiments of the presentdisclosure. System 900 includes a device 905, which may be or include aprinted circuit board to connect or physically support variouscomponents. Device 905 includes a memory array 100-b, which may be anexample of memory array 100 described with reference to FIG. 1 and FIG.6. Memory array 100-b may contain memory controller 140-b and memorycells 105-e, which may be examples of memory controller 140 describedwith reference to FIGS. 1 and 6 and memory cells 105 described withreference to FIGS. 1, 2, 4, 6, and 8. Device 905 may also include aprocessor 910, BIOS component 915, peripheral component(s) 920,input/output control component 925, and DL voltage adjust component 940.DL voltage adjust component 940 may be an example of DL voltage adjustcomponent 830 as described with reference to FIG. 8. The components ofdevice 905 may be in electronic communication with one another throughbus 930.

Processor 910 may be configured to operate through memory controller140-b. In some cases, processor 910 may perform the functions of memorycontroller 140 described with reference to FIGS. 1 and 6. In othercases, memory controller 140-b may be integrated into processor 910.Processor 910 may be a general-purpose processor, a digital signalprocessor (DSP), an application-specific integrated circuit (ASIC), afield-programmable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or itmay be a combination of these types of components, and processor 910 mayperform various functions described herein, including supporting aground reference scheme for a memory cell. Processor 910 may, forexample, be configured to execute computer-readable instructions storedin to cause device 905 perform various functions or tasks.

BIOS component 915 may be a software component that includes a basicinput/output system (BIOS) operated as firmware, which may initializeand run various hardware components of system 900. BIOS component 915may also manage data flow between processor 910 and the variouscomponents, e.g., peripheral components 920, input/output controlcomponent 925, etc. BIOS component 915 may include a program or softwarestored in read-only memory (ROM), flash memory, or any othernon-volatile memory.

Peripheral component(s) 920 may be any input or output device, or aninterface for such devices, that is integrated into device 905. Examplesmay include disk controllers, sound controller, graphics controller,Ethernet controller, modem, USB controller, a serial or parallel port,or peripheral card slots, such as peripheral component interconnect(PCI) or accelerated graphics port (AGP) slots.

Input/output control component 925 may manage data communication betweenprocessor 910 and peripheral component(s) 920, input devices or outputdevices received via input 935 or output 945. Input/output controlcomponent 925 may also manage peripherals not integrated into device905. In some cases, input/output control component 925 may represent aphysical connection or port to the external peripheral.

Input 935 may represent a device or signal external to device 905 thatprovides input to device 905 or its components. This may include a userinterface or interface with or between other devices. In some cases,input 935 may be a peripheral that interfaces with device 905 viaperipheral component(s) 920 or may be managed by input/output controlcomponent 925.

Output 945 may be implemented as a device or signal external to device905 configured to receive output from device 905 or any of itscomponents. Examples of an output device may include a display, audiospeakers, a printing device, another processor or printed circuit board,etc. In some cases, output 945 may be a peripheral that interfaces withdevice 905 via peripheral component(s) 920 or may be managed byinput/output control component 925.

The components of memory controller 140-b, device 905, and memory array100-b may be made up of circuitry designed to carry out their functions.This may include various circuit elements, for example, conductivelines, transistors, capacitors, inductors, resistors, amplifiers, orother active or passive elements, configured to carry out the functionsdescribed herein.

FIG. 10 shows a flowchart illustrating a method 1000 for utilizing aground reference scheme for a memory cell in accordance with variousembodiments of the present disclosure. The operations of method 1000 maybe implemented by a memory array 100, as described with reference toFIGS. 1-9. For example, the operations of method 1000 may be performedby a memory controller 140, as described with reference to FIGS. 1, 8,and 9. In some examples, a memory controller 140 may execute a set ofcodes to control the functional elements of the memory array 100 toperform the functions described below. Additionally or alternatively,the memory controller 140 may perform features of the functionsdescribed below using special-purpose hardware.

At block 1005, the method may include applying a positive voltage to aferroelectric capacitor of the ferroelectric memory cell, wherein theferroelectric capacitor is in electronic communication with a digitline. In certain examples, the operations of block 1005 may be performedor facilitated by the biasing component 810, as described with referenceto FIG. 8.

At block 1010, the method may include determining that a voltage of thedigit line has reached a threshold in response to the positive voltagebeing applied. In certain examples, the operations of block 1010 may beperformed or facilitated by the timing component 815, as described withreference to FIG. 8. In some cases, determining that the voltage of thedigit line has reached the threshold includes determining that thevoltage of the digit line has reached a threshold voltage. Additionallyor alternatively, determining that the voltage of the digit line hasreached the threshold may include determining that a rate of change ofthe voltage of the digit line has reached the threshold.

At block 1015, the method may include applying a negative voltage to theferroelectric capacitor after the voltage of the digit line reaches thethreshold. In certain examples, the operations of block 1015 may beperformed or facilitated by the biasing component 810, as described withreference to FIG. 8. In some cases, a magnitude of the negative voltageapplied to the ferroelectric capacitor is based at least in part on thethreshold. Determining that the voltage of the digit line has reachedthe threshold may include determining that the positive voltage has beenapplied for a predetermined duration. The predetermined duration may bebased at least in part on at least one of a characteristic of theferroelectric capacitor, a characteristic of the digit line, a timingassociated with reading or writing to the ferroelectric memory cell, orany combination thereof. In some cases, the method includes comparingthe voltage of the digit line to a ground reference after the negativevoltage is applied. In some cases, a logic value of the ferroelectricmemory cell is determined based at least in part on the comparison ofthe voltage of the digit line to the ground reference.

FIG. 11 shows a flowchart illustrating a method 1100 for utilizing aground reference scheme for a memory cell in accordance with variousembodiments of the present disclosure. The operations of method 1100 maybe implemented by a memory array 100, as described with reference toFIGS. 1-9. For example, the operations of method 1100 may be performedby a memory controller 140, as described with reference to FIGS. 1, 8,and 9. In some examples, a memory controller 140 may execute a set ofcodes to control the functional elements of the memory array 100 toperform the functions described below. Additionally or alternatively,the memory controller 140 may perform features of the functionsdescribed below using special-purpose hardware.

At block 1105, the method may include applying a first voltage to aferroelectric capacitor of the ferroelectric memory cell, wherein theferroelectric capacitor is in electronic communication with a digitline. In certain examples, the operations of block 1105 may be performedor facilitated by the biasing component 810, as described with referenceto FIG. 8. In some examples, the method may include selecting theferroelectric memory cell for a read operation by activating a firstselection component that is in electronic communication with theferroelectric capacitor and the digit line; and activating a secondselection component that is in electronic communication with thereference capacitor and the digit line. In some cases, the firstselection component and the second selection component are activatedprior to applying the first voltage.

At block 1110, the method may include applying a second voltage to areference capacitor that is in electronic communication with the digitline, wherein the second voltage is an inverse of the first voltage andis applied based at least in part on applying the first voltage. Incertain examples, the operations of block 1110 may be performed orfacilitated by the biasing component 810, as described with reference toFIG. 8. In some examples, the first voltage and second voltages areapplied substantially simultaneously. In some cases, the first selectioncomponent and the second selection component are activated prior toapplying the first voltage and/or the second voltage. In some examples,the method may include virtually grounding the digit line beforeapplying the first voltage or the second voltage. In some examples, themethod includes comparing a voltage of the digit line to a groundreference after the second voltage is applied to the referencecapacitor. Determining a logic value of the ferroelectric memory cellmay be based at least in part on the comparison of the voltage of thedigit line to the ground reference

Thus, methods 1000 and 1100 may provide for utilizing a ground referencescheme. It should be noted that methods 1000 and 1100 describe possibleimplementations, and the operations and steps may be rearranged orotherwise modified such that other implementations are possible. In someexamples, features from two or more of the methods 1000 and 1100 may becombined.

The description herein provides examples, and is not limiting of thescope, applicability, or examples set forth in the claims. Changes maybe made in the function and arrangement of elements discussed withoutdeparting from the scope of the disclosure. Various examples may omit,substitute, or add various procedures or components as appropriate.Also, features described with respect to some examples may be combinedin other examples.

The description set forth herein, in connection with the appendeddrawings, describes example configurations and does not represent allthe examples that may be implemented or that are within the scope of theclaims. The terms “example” and “exemplary,” as used herein, mean“serving as an example, instance, embodiment, or illustration,” and not“preferred” or “advantageous over other examples.” The detaileddescription includes specific details for the purpose of providing anunderstanding of the described techniques. These techniques, however,may be practiced without these specific details. In some instances,well-known structures and devices are shown in block diagram form inorder to avoid obscuring the concepts of the described examples.

In the appended figures, similar components or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. When the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

Information and signals described herein may be represented using any ofa variety of different technologies and techniques. For example, data,instructions, commands, information, signals, bits, symbols, and chipsthat may be referenced throughout the above description may berepresented by voltages, currents, electromagnetic waves, magneticfields or particles, optical fields or particles, or any combinationthereof. Some drawings may illustrate signals as a single signal;however, it will be understood by a person of ordinary skill in the artthat the signal may represent a bus of signals, where the bus may have avariety of bit widths.

As used herein, the term “virtual ground” refers to a node of anelectrical circuit that is held at a voltage of approximately zero volts(0V) but that is not directly connected with ground. Accordingly, thevoltage of a virtual ground may temporarily fluctuate and return toapproximately 0V at steady state. A virtual ground may be implementedusing various electronic circuit elements, such as a voltage dividerconsisting of operational amplifiers and resistors. Otherimplementations are also possible. “Virtual grounding” or “virtuallygrounded” means connected to approximately 0V.

The term “electronic communication” refers to a relationship betweencomponents that supports electron flow between the components. This mayinclude a direct connection between components or may includeintermediate components. Components in electronic communication may beactively exchanging electrons or signals (e.g., in an energized circuit)or may not be actively exchanging electrons or signals (e.g., in ade-energized circuit) but may be configured and operable to exchangeelectrons or signals upon a circuit being energized. By way of example,two components physically connected via a switch (e.g., a transistor)are in electronic communication regardless of the state of the switch(i.e., open or closed). The term “isolated” refers to a relationshipbetween components in which electrons are not presently flowing betweenthe components. For example, two components physically connected by aswitch may be isolated from each other when the switch is open.

The devices discussed herein, including memory array 100, may be formedon a semiconductor substrate, such as silicon, germanium,silicon-germanium alloy, gallium arsenide, gallium nitride, etc. In somecases, the substrate is a semiconductor wafer. In other cases, thesubstrate may be a silicon-on-insulator (SOI) substrate, such assilicon-on-glass (SOG) or silicon-on-sapphire (SOP), or epitaxial layersof semiconductor materials on another substrate. The conductivity of thesubstrate, or sub-regions of the substrate, may be controlled throughdoping using various chemical species including, but not limited to,phosphorous, boron, or arsenic. Doping may be performed during theinitial formation or growth of the substrate, by ion-implantation, or byany other doping means.

A transistor or transistors discussed herein may represent afield-effect transistor (FET) and comprise a three terminal deviceincluding a source, drain, and gate. The terminals may be connected toother electronic elements through conductive materials, e.g., metals.The source and drain may be conductive and may comprise a heavily-doped,e.g., degenerate, semiconductor region. The source and drain may beseparated by a lightly-doped semiconductor region or channel. If thechannel is n-type (i.e., majority carriers are electrons), then the FETmay be referred to as a n-type FET. If the channel is p-type (i.e.,majority carriers are holes), then the FET may be referred to as ap-type FET. The channel may be capped by an insulating gate oxide. Thechannel conductivity may be controlled by applying a voltage to thegate. For example, applying a positive voltage or negative voltage to ann-type FET or a p-type FET, respectively, may result in the channelbecoming conductive. A transistor may be “on” or “activated” when avoltage greater than or equal to the transistor's threshold voltage isapplied to the transistor gate. The transistor may be “off” or“deactivated” when a voltage less than the transistor's thresholdvoltage is applied to the transistor gate.

The various illustrative blocks, components, and modules described inconnection with the disclosure herein may be implemented or performedwith a general-purpose processor, a DSP, an ASIC, an FPGA or otherprogrammable logic device, discrete gate or transistor logic, discretehardware components, or any combination thereof designed to perform thefunctions described herein. A general-purpose processor may be amicroprocessor, but in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine. Aprocessor may also be implemented as a combination of computing devices(e.g., a combination of a DSP and a microprocessor, multiplemicroprocessors, one or more microprocessors in conjunction with a DSPcore, or any other such configuration).

The functions described herein may be implemented in hardware, softwareexecuted by a processor, firmware, or any combination thereof. Ifimplemented in software executed by a processor, the functions may bestored on or transmitted over as one or more instructions or code on acomputer-readable medium. Other examples and implementations are withinthe scope of the disclosure and appended claims. For example, due to thenature of software, functions described above can be implemented usingsoftware executed by a processor, hardware, firmware, hardwiring, orcombinations of any of these. Features implementing functions may alsobe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations. Also, as used herein, including in the claims, “or” as usedin a list of items (for example, a list of items prefaced by a phrasesuch as “at least one of” or “one or more of”) indicates an inclusivelist such that, for example, a list of at least one of A, B, or C meansA or B or C or AB or AC or BC or ABC (i.e., A and B and C).

Computer-readable media includes both non-transitory computer storagemedia and communication media including any medium that facilitatestransfer of a computer program from one place to another. Anon-transitory storage medium may be any available medium that can beaccessed by a general purpose or special purpose computer. By way ofexample, and not limitation, non-transitory computer-readable media cancomprise RAM, ROM, electrically erasable programmable read only memory(EEPROM), compact disk (CD) ROM or other optical disk storage, magneticdisk storage or other magnetic storage devices, or any othernon-transitory medium that can be used to carry or store desired programcode means in the form of instructions or data structures and that canbe accessed by a general-purpose or special-purpose computer, or ageneral-purpose or special-purpose processor.

Also, any connection is properly termed a computer-readable medium. Forexample, if the software is transmitted from a website, server, or otherremote source using a coaxial cable, fiber optic cable, twisted pair,digital subscriber line (DSL), or wireless technologies such asinfrared, radio, and microwave, then the coaxial cable, fiber opticcable, twisted pair, digital subscriber line (DSL), or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. Disk and disc, as used herein, include CD, laserdisc, optical disc, digital versatile disc (DVD), floppy disk andBlu-ray disc where disks usually reproduce data magnetically, whilediscs reproduce data optically with lasers. Combinations of the aboveare also included within the scope of computer-readable media.

The description herein is provided to enable a person skilled in the artto make or use the disclosure. Various modifications to the disclosurewill be readily apparent to those skilled in the art, and the genericprinciples defined herein may be applied to other variations withoutdeparting from the scope of the disclosure. Thus, the disclosure is notto be limited to the examples and designs described herein but is to beaccorded the broadest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A method of operating a ferroelectric memorycell, comprising: applying a positive voltage to a ferroelectriccapacitor of the ferroelectric memory cell, wherein the ferroelectriccapacitor is in electronic communication with a digit line; determiningthat a rate of change of a voltage of the digit line has reached athreshold in response to the positive voltage being applied; andapplying a negative voltage to the ferroelectric capacitor after thevoltage of the digit line reaches the threshold.
 2. The method of claim1, further comprising: comparing the voltage of the digit line to aground reference after the negative voltage is applied.
 3. The method ofclaim 2, further comprising: determining a logic value of theferroelectric memory cell based at least in part on the comparison ofthe voltage of the digit line to the ground reference.
 4. The method ofclaim 1, wherein a magnitude of the negative voltage applied to theferroelectric capacitor is based at least in part on the threshold. 5.The method of claim 1, further comprising: determining that the voltageof the digit line has reached a second threshold based at least in parton determining that the positive voltage has been applied for apredetermined duration.
 6. The method of claim 5, wherein thepredetermined duration is based at least in part on at least one of: acharacteristic of the ferroelectric capacitor, a characteristic of thedigit line, a timing associated with reading or writing to theferroelectric memory cell, or any combination thereof.
 7. The method ofclaim 1, further comprising: determining that the voltage of the digitline has reached a second threshold based at least in part ondetermining that the voltage of the digit line has reached a thresholdvoltage.
 8. The method of claim 1, wherein applying the positive voltageto the ferroelectric capacitor comprises: virtually grounding the digitline; applying the positive voltage to a plate line in electroniccommunication with the ferroelectric capacitor; and applying a secondvoltage to a word line in electronic communication with theferroelectric capacitor.
 9. The method of claim 1, wherein applying thenegative voltage to the ferroelectric capacitor comprises: applying thenegative voltage to a plate line in electronic communication with theferroelectric capacitor.
 10. The method of claim 1, wherein determiningthat the rate of change of the voltage of the digit line comprisesdetermining that the rate of change is less than 10 mV/ns.
 11. Themethod of claim 1, wherein determining that the rate of change of thevoltage of the digit line comprises determining that the voltage of thedigit line is within a percent range of an expected settling voltage.12. An electronic memory apparatus, comprising: a ferroelectric memorycell that comprises a ferroelectric capacitor in electroniccommunication with a digit line; and a controller in electroniccommunication with the ferroelectric memory cell and operable to:connect a positive voltage source to the ferroelectric capacitor; anddetermine that a rate of change of a voltage of the digit line hasreached a threshold; connect a negative voltage source to theferroelectric capacitor after the voltage of the digit line reaches thethreshold.
 13. The electronic memory apparatus of claim 12, wherein thecontroller is operable to: compare the voltage of the digit line to aground reference after a negative voltage is applied to theferroelectric capacitor.
 14. The electronic memory apparatus of claim13, wherein the controller is operable to: determine a logic value ofthe ferroelectric memory cell based at least in part on a comparison ofthe voltage of the digit line to the ground reference.
 15. Theelectronic memory apparatus of claim 12, wherein the controller isoperable to: determine that a voltage of the digit line has reached asecond threshold in response to a positive voltage applied to theferroelectric capacitor.
 16. The electronic memory apparatus of claim15, wherein the controller is operable to: determine that a voltage ofthe positive voltage source has been applied to the ferroelectriccapacitor for a predetermined duration; and determine that the voltageof the digit line has reached the second threshold based at least inpart on determining that the voltage of the positive voltage source hasbeen applied for the predetermined duration.
 17. The electronic memoryapparatus of claim 16, wherein the predetermined duration is based atleast in part on at least one of: a characteristic of the ferroelectriccapacitor, a characteristic of the digit line, a timing associated withreading or writing to the ferroelectric memory cell, or any combinationthereof.
 18. The electronic memory apparatus of claim 12, furthercomprising: a plate line in electronic communication with theferroelectric capacitor, wherein the controller is operable to: applyinga positive voltage to the plate line by connecting the positive voltagesource to the ferroelectric capacitor.
 19. The electronic memoryapparatus of claim 12, further comprising: a plate line in electroniccommunication with the ferroelectric capacitor, wherein the controlleris operable to: apply a negative voltage to the plate line by connectingthe negative voltage source to the ferroelectric capacitor.
 20. Theelectronic memory apparatus of claim 12, wherein a voltage of thenegative voltage source is based at least in part on the threshold.